Ball mills became the dominant fine-grinding machine in the 20th century. By 1900, the ball mill patent litigation in France had ended, and inventions to improve fine-grinding processes were in full flood. Inventing a machine is one thing, selling it another. One event that went far to establish the credibility of wet ball mills as fine-grinding machines was the installation, in 1910, of 64 Hardinge conical pebble mills in the retreatment plant of the Calumet and Hecla copper mine in Michigan’s Upper Peninsula.
Harlowe Hardinge was a consulting mining engineer who observed that large and small rocks moved at different rates when moving down an incline. It occurred to him that if a mill had a cylindrical section at the feed end attached to a long conical section at the discharge end, the rocks that were the grinding media would arrange themselves in the mill according to size, with the coarse media in the cylindrical section and the fine media spread along the conical section (Crushing and Grinding 1931).
This would improve the efficiency of grinding by breaking the coarse feed particles early in the mill, with coarse media and fine particles later in the mill with fine media. The same concept is used with classifying liners today. Figure 7.10 shows the mill that Hardinge built.
The Hardinge mill had trials in Pennsylvania and Missouri with pebbles as the grinding media, but it was at the renowned Calumet and Hecla Mine where it established a reputation as the most suitable machine for grinding hard metalliferous ores. Native copper had been mined in this Lake Superior region since prehistoric times (Drier and Du Temple 1961). The deposits were large, and several companies were mining ores containing native copper and primary copper sulphides in the 19th and 20th centuries. Milling practice at the Lake Superior mines in the 1850s was to break large boulders containing the copper by hand and then to use Blake jaw crushers and stamp mills for further breakage and to concentrate the copper minerals on jigging tables. This practice continued for decades, and the recovery of copper was always poor.
In 1908, the management of the Calumet and Hecla Mine, which had become the main operating company in the region, installed a processing plant to extract copper from 37 million tons of tailings that had accumulated over 50 years. These tailings contained 5 kg (11 lb; 0.5%) of copper per ton of ore (Benedict 1955), but they were hard and tough and had to be ground from 6 mm to -0.2 mm. It was the practice at the time when building new plants to run a competitive test between machines from rival manufacturers. A 12-month trial was carried out with three different mills—a tube mill, a Chilean mill, and the Hardinge conical mill. The demanding test was won by the Hardinge mill, and the result was that 64 open-circuit mills were installed in a new tailings treatment plant, each 2.6 m in diameter with a 0.5-m cylindrical section and using flint pebbles as grinding media. They are shown in Figure 7.11.
The use of the Hardinge mill increased copper recovery immediately to 75%, and, when steel balls replaced the flint pebbles a few years later and increased the energy that could be used in grinding, this recovery rose to 90%. By then, flotation was also being used, so how much of the total increase was the result of flotation and how much resulted from finer grinding is not known. It is sufficient to say that both the tumbling mill and the type of media used contributed to a substantial increase in the recovery of copper.